1. Field of the Invention
The present invention refers to stirrups for reinforcement of load bearing structural elements, and in particular for reinforcing concrete load bearing building elements, such as columns, shear walls, beams, slabs, footings, lintels, piles. The invention refers also to a method for reinforcing structural elements as well as to these elements.
2. Description of the Prior Art
Stirrups and ties constitute one of the most critical factors of quality and antiseismic strength of buildings. Essential factors for the liability of stirrups are the proper hooks at their ends and the bend diameter at corners. The hooks at the end of the conventional stirrups are absolutely necessary for ensuring the proper functioning of the stirrup or tie in case of a very strong earthquake, when the spalling of the concrete occurs and when the hooks is the only remaining anchorage mechanism.
The following stirrups are used in building industry today:
i) Individual stirrups 8, which may be of various forms, such as described in FIG. 1. For individual stirrups it is essential to be fastened in a plurality of points to the principal reinforcement rods 1 of the reinforcement as well as to the woodform. Thus their assembly is complicated and has a high cost. The individual stirrups 8 comprise hooks 6, for anchoring the stirrups to the load-bearing element of the structure.
ii) "Mantles", i.e. stirrup cages made of prefabricate welded meshes (see FIG. 2). These are made of standardised welded meshes in suitable machines. The partial replacement of common stirrups by the "mantles" or "stirrup cages" was the first attempt to transform the painful task of reinforcing the load bearing elements of the structure into an industrial process. However the manufacture of the mantles is done in two phases, and only part of the process may become an industrial one: The first phase is an industrial process aiming in the production of plane meshes, such as shown in FIG. 3, from steel rolls using huge machines. During the second phase the meshes are almost manually assembled to form stirrup cages. The production of "mantles" have the following limitations: a) it is difficult to manufacture compound stirrup shapes by analysing them in simple rectangular shapes, b) it is impossible to increase or decrease the spacing of the stirrups resulting in superfluous weight of the reinforcement, c) it is expensive to transport them due to the size of the cages, d) it is difficult to manufacture double hooks, which is a necessity in antiseismic structures, and e) there is a danger of buckling of the vertical binding bars in case of an earthquake.
iii) Circular or orthogonal spiral stirrups such as disclosed in EP-A-0152397. Numerous experiments have been executed with circular spirals, which proved that if the spacing of the windings, i.e. the pitch, is kept below a minimum distance, the spirals are actually functioning like steel closed mantles, whose strength is increased due to the presence of triaxial stress system. The spiral stirrups currently known are appropriate only for reinforcing columns with rectangular cross-section. Further they are uneconomical because of the constant spacing between windings, which is determined by the shear level at the most critical region of the member. They also present problems in manufacturing and difficulties in placing them by the skilled workmen, because of the excessive weight in cases of strongly reinforced columns with many sides.